The present invention relates to a process for preparing pesticides which release hydrogen phosphide gas.
It is known that under the influence of water, certain phosphides of alkaline earth metals and earth metals such as calcium, magnesium and aluminum phosphide, will release hydrogen phosphide, a gas suitable for combatting pests. The above phosphides may be added in various forms, e.g., in bags, as tablets or other molded bodies, to the goods to be treated. Under the influence of moisture, hydrolysis will occur and phosphine is formed in an amount and at a rate useful for killing various pests.
The reaction rate is normally controlled by increasing the hydrophobicity of the phosphide, e.g., with hard paraffin, metal stearates, waxes, paraffin oils and the like. The use of such compounds is, however, disadvantageous as residues of the pesticidal composition still containing minor amounts of unreacted phosphide will not be wetted and decomposed when treated with water. Instead, the residues will float on the water surface and cannot even be wetted by stirring. Thus it is extremely difficult to decompose the phosphide under controlled conditions. There results the danger that decomposition may take place under conditions where the release of toxic hydrogen phosphide could be dangerous or even fatal.
Attempts have been made to avoid this undesirable result by employing small quantities of polyethylene oxides (PEO); see West German Offenlegungschrift No. 2,443,333. As disclosed therein, the known film-forming PEO compounds will influence the decomposition rate in the desired extent without impairing the wettability of the composition.
Preparation of the pesticides as disclosed in the above publication is effected by mixing the individual components including PEO with exclusion of moist air, and subsequently compressing the mixture in order to obtain molded bodies, such as tablets, pellets and the like. Such molding normally requires the use of compacting agents such as organic fatty acids.
Experiments have, however, shown that the desired delay in the release of hydrogen phosphide gas can only be accomplished to a very limited degree when using PEO so that the handling of the pesticides may still be dangerous unless careful precautions are taken.
Moreover, technical difficulties are encountered in preparing molded bodies from such mixtures. The mixtures often have unsatisfactory flowability leading to variations in the weight of molded bodies and other disadvantages.
Molded bodies containing only small amounts of PEO are of moderate strength and quality.
Another important disadvantage is that molded bodies prepared in the described way have a tendency to self-ignite on contact with water.
It is an object of the present invention to overcome the above disadvantages and to provide pesticides which, with regard to preparation and use, comply with all essential criteria.